Liquid supply pumps are used extensively in medical and other fields. In medical fields, for example, the use of intravenous (IV) pumps for delivery of fluids, such as medications and nutrient solutions, has been a wide-spread practice in hospitals. IV pumps have gained widespread acceptance because they are able to deliver IV fluids under accurate and tightly controlled conditions so that medications and the like can be delivered intravenously to a patient wherein deviations from a desired delivery rate can have harmful consequences.
An IV pump device is often provided with a pumping mechanism that is adapted to accept a cassette containing a pumping chamber. The cassette is typically designed for one use only, and needs to be economically manufactured to reduce its cost. The cassette is typically activated by a reciprocatory (e.g., peristaltic) driving force of the pumping mechanism and has a fluid inlet for connecting to a tube leading to the supply container and a fluid outlet for connecting to a tube that delivers the IV fluid to the patient.
For control and monitoring purposes, it is desirable to measure the fluid pressure within the disposable IV cassette. For example, the fluid pressure signal can be used to detect, inter cilia, an empty supply bottle, an occluded intake or outlet path, a bottle-channel association, the fluid level in the bottle and the flow resistance of the fluid pathway. One challenge is to provide a pressure sensing system for measuring the fluid pressure within the cassette that is both economical and accurate. Ideally the sensor accurately measures both positive and negative pressures, negative pressures commonly arising due to elevation of the patient of container with respect to the sensor element. High resolution on the order of 1 mmHg is needed as well for the purposes mentioned above.
Conventionally, the fluid pressure within a cassette is measured by a contact measurement method in which the cassette or an object attached to the cassette physically contacts a sensing arrangement (e.g., a resistive strain gauge force sensor) to exert a contact pressure/force on the sensing arrangement. In such contact-based pressure sensing systems, the sensing arrangement is typically intentionally preloaded with a positive pressure/force such as exerted by a deformed tubing wall in order to artificially bias a zero-pressure point so that a negative pressure can be measured. One problem with such a positive-biasing scheme with a preloaded sensing arrangement is that the preloading force can diminish over time due to stress relaxation resulting in the related biasing point drifting downward over time. This may cause underestimation of true fluid pressure.